Alternating-current hoist control



Dec. 12, 1950 w. R. WICKERHAM ALTERNATING-CURRENT HOIST CONTROL 2Sheets-Sheet 1 Filed May 24, 1949 INVENTOR William R. Wickerhom.

ATT RNEY Patented Dec. 12, 1950 ALTERNATING-CURRENT HOIST CONTROLWilliam R. Wickerham, Buffalo, N. Y., assignor to Westinghouse ElectricCorporation, East Pittsburgh, Pa., a corporation of PennsylvaniaApplication May 24, 1949, Serial No. 95,024

7 Claims. 1

My invention relates to control systems for alternating-current motorsand, in a more specific aspect, to control systems for applications,such as on hoists, that require operating under overhauling motor loads.

It is an object of my invention to provide a motor control system of thetype mentioned that permits adding one or several speed-torque steps tothose obtainable by secondary resistance control alone and that, forthis purpose, requires simpler control devices than those of knownsystems.

Another, more specific object of the invention is to provide an A.-C.motor control system capable of selectively handling an overhauling loadunder single-phase braking performance and three-phase drivingperformance while preventing or substantially reducing the transitoryhigh currents apt to occur in known systems when switching between thesetwo performance characteristics.

The invention also aims at devising a control system in accordance withthe foregoing objects that, in addition, is automatically effective tolimit the motor speed when the system is set for driving a load in theoverhauling direction and that, for this purpose, requiresspeed-limiting means of extremely simple design and a minimum of relaysor contactors.

These and other objects as well as the means,

specified by the annexed claims, for achievingthese objects inaccordance with my invention will be apparent from the followingdescription in conjunction with the drawings which show a semi-automatichoist control system.

Fig. lillustrates a circuit diagram of the complete hoist controlsystem. Figs. 2 to 6 show straight-line circuit diagrams of the powerconnections obtained at different settings of the master controller inthe system of Fig. l; and

Fig. 7 is a coordinate diagram exemplifying typical speed-torquecharacteristics of the motor in the same system.

In Fig. 1 the line terminals of the system are denoted by Li, L2 and L3.The hoist motor M is of the wound rotor type and has three primaryterminals Tl, T2 and T3 and three secondary terminals SI, S2 and S3. Thehoist machinery or load to be driven or ibraked by the motor isschematically shown at ID. The hoist motor is equipped with a normallyset friction brake II which is magnetically releasable by a brake collI2.

The connection of the motor terminals Ti, T2 and T3 with the lineterminals is controlled by a master controller MC and a contactor C. Themain contacts [3, l4 and I5 and a self-holding contact I6 of contactor Care controlled by a coil H whose circuit extends between line terminalsL2 and L3 through a contact device of the master controller MC. Themaster controller is represented in developed form and exemplified by adrum contactor although it will be understood that controllers of othertypes, such as cam controllers or push-button controllers, may be usedinstead. The illustrated master controller is equipped with contactfingers such as those denoted by F, and contact segments such as thosedenoted by S. The master controller is selectively adjustable between anoff position, five hoisting positions, and three lowering positions.

Connected to the secondary terminals, Si, S2, S3 of motor M is aresistance circuit composed of resistors denoted by l8 through 26. Theresisters 20, 23 and 26 are connected with the master controller so thatthey are shorted out of the secondary motor circuit when the mastercontroller is in the first position lowering or in the higher hoistingpositions. The resistance of the secondary motor circuit is furthercontrolled by the contacts 29 and 30 of a contactor A whose coil 3| alsoactuates a contact 32. The coil circuit for contactor A is controlled bythe master 7 controller MC and is also under control by a contact 33 ofa locking relay R whose coil 34 controls also a contact 35.

The system is equipped with a speed limiting relay H which has two coils36 and 31 for controlling a contact 38 in the coil circuit of relay R.Coil 36 is connected through a rectifier 39 across the resistors l8, l9and 2D and hence is energized in accordance with voltage from thesecondary motor circuit. The coil 31 is connected through a rectifier4'0 under control by the master controller to an energizing circuit sothat coil 3'! is excited in all positions of the master controllerexcept in the second and third lowering positions. Unless the mastercontroller is adjusted to the second or third lowering position, thecoil 31 is excited and maintains the contact 38 in normally openposition. Coil 3B is sufiiciently excited to hold the contact 38 openwhenever the motor speed is below a critical value closely belowsynchronous speed. When the motor is running at a speed below thecritical value while the master controller is being moved from positionI lowering to position 2 lowering, the holding coil 31 becomesineffective, but since at that time the coil 33 is excited bysufficiently high 3 voltage the contact 38 remains nevertheless open andcan drop into closed position only when the accelerating motor exceeds agiven speed somewhat below synchronism. As will be explained in thefollowing, the closing of contact 33 under the just mentioned conditionshas the effect of energizing the relay 3 which in t n-n causes thecontactorj l to pick up and to readjust" the con: trol system so as toautomatically limit the mo tor speed. The relay R, when thus picking upseals itself in and thus ope as a blocking means which prevents any Ithe speed limit relay H ironi having a cont efiect as long as the mastercontroller the second lowering position. In the effectiveness of thespeed l ir egrel y,

is definitely limited to the deeireales enseet tilepredetermined highsubsynchronous speed andho further control effect can take place whenthe secondary motor ,voltage inoreases due to the motor s eem-es itsrono'us speed The system is fi' rther ped with a resis 4 l which has oneend perm'a efitly' connected with the primary meter terminal TII. 'Ihefother end oi resistor M is oonnected',f1inder control by the a mastercontroller .340 either with the 1ine..,ter minal' Ll or v'vith the otorterminal T2 dependmg upon the slecte'd osition of the master con;trollei. In the illustrated. example, the resistor 4| lies. acrn'isfs.motor term s I1 and TZ only when the master contrqn position. In thesame posit th'e' line terminals 2 and L3. ar connected with motel:terminate;

T3 and T2; resbe'qtivei w n e thjeune terminal L Lis' disconnected fromthe motor, v With this adjustment of the master co'n'troller the motorreceives' singlephase excitation, and the resistor 4f establishes aShort "i'rfiuit b'et'ween tvv niotor terminals order, 0 produc a brakingfield. The series connect of resistor between niotorj terminal TI andline terminal Ll occurs only when the master controller MC is in thesecond or third lower position-or in the first hoist ng DOS} on. A thesame time; the remainin' two .line t rm'inals' are Connected to therelearning two" primary terminals f t v meter. Under tliesei conditionsthe motor receives three}; phase xcitation Willo ,1 duegto theasymmetrical arrangement of the resistor l-l is unbalanoedthns producinga lower torque tha n obtained under hoist ng and Waring operation,

Assumethatt e line terminals nj z iris e v i v jed due to the closing orimam switch (not illustrated) and that tne mast r controller MC is, in.the illu$tral17ed' o'ff' position Then the contaotor C is picked up; b'arise j energ zed through the master ntrollel'f n; ter tent ner is movfrom the position; the e nteetm c remeinsfiierea iip'ufitil er operates-01" tl'i' first lowering I either the line terminals are deenergized orthe line voltage drops below a safe value. In either event the contactorC will again pick up only if the master controller is first placed inthe off position. The holding coil 37 of the speed limit relay H is alsoenergized from line terminals LI and LZ,thr,9ugh the mas r qqn tqller Mand through the ractifier 4&1 Qof sequiently, the contact 38 is open;When the master controller is moved to the first hoisting position, itconncts line terminal L2 with motor terminal T2, are lin term'mer L3with motor terminal T3. connected through the master terminal Tl inseries with the between motor to minals T2 and T3 so that the were islifted? This is also the case in all other hoisting and loweringpositions of the master controller. All resistors 18 through 26 are con-20 nectedin the secondary rr'iotor circuit The power o e ofnsl h th m tt us es a ishes.

i wiilh' 'if I ii efiinb qii excitation caused by the series resistor Mthe motor is energized for low-torque 01' t the hoisting direction. Thec0rrespond1ng l e A t s si torque characteristic is exemplified bycfirve HI Eigfi. .7 a

Wh pthe ma er int an t is i e dt 2 t. i eceht sis-i icu i ithe esi orat) it o that terminals n and 1 ,1 are direotI y nected with each other.No other change 060}; s; ,1 th 5 .m so thattherqv o ne o 9 b ain n arein ac fdane ,i lF a 3? h motor receives balanced thre'r'lih-as'eetatioi'i i re d t l n i ins l a was em? with the speed torque: characterstic H2 mIFig'. 7.

At v point 3 hoist, seco dary resistors" 23' and Q6 are shorted in'flthniajster controller thus"- c n f the. me e t r i p give sr' d- Thecorresponding motor characteristic ex: emplified hy curye H3 inFig.7.

At point 4 hoist the resistor iilis" alsdshortecl in the mastercontroller; resulting in a speed s aster controller, the ir ui i 'j'I, rmaster controller: Contacts 2'9 and (iii of can tactor A short therenewa s I9, 22? and 2 so that only the resistors l8, 2l and 24 remainin the motor oircuit. The corresponding s eed t rque characteristic fortorqu' conditions is typified by curve HSinFi'gj. '7.

Starting again from the off position, mead just nien t of controller topoint I lowering has the iQll'. "ins; effects? Controller ifrjcconnect}; line term'i'naLLZ toirnotor terminal" T3, andline terminal L3to moto'r terminal T2; remain Ll rernains diseoml e' fed, and est or erreconn cted acfr ssin or terminals'Tl and T3, Controller v o also shortsthe secondary ren tors 2Q, 23 and 261.; The rfiotoif cjiruit' thusestablished and motor terminal TI. The motor now receives unbalancedthree-phase energization for lowtorque operation in the loweringdirection. Controller MC disconnects the coil 37 of speed limiting relayH from its current supply. However, as long as the lowering speedremains appreciably below the synchronous value, coil 36 receivessufficiently high voltage from across the secondary resistors |8--|9-20to keep relay H picked up. The motor circuit is shown in Fig. 5, and theperformance of the motor is exemplified by the speed torquecharacteristic D2 in Fig. 7.

However, if the speed of an overhauling load is permitted to exceed agiven high value slightly below synchronism, the voltage acrossresistors |8-|9--2|l drops below the critical pick up value for relay H.The relay drops out and its contact 38 closes the circuit for coil 34 inrelay R. Relay R closes its holding contact 35 while contact 33 providesexcitation for the coil 3| of contactor A. Contact 32 of contactor Ashorts the resistor 4|, and contacts 29 and 30 short the secondaryresistors except resistors l8, 2| and 24. Consequently, when the speedlimit relay H responds, the motor circuit is automatically changed fromthe condition represented in Fig. 5 to that shown in Fig. 6. The motornow receives balanced three-phase excitation under such secondaryresistance condition that the motor tends to run near synchronous speedwithin the available range of torques. Thus the motor speed isautomatically limited to a value near synchronism in accordance with thetypical characteristic D3 in Fig. 7. It will be noted that once therelay R has responded, its coil circuit remains closed through theself-holding contact 35 as long as the master controller is in position2 lowering. Consequently, the relay R operates as a blocking means andrenders the relay H ineffective. In this manner the relay H is preventedfrom the affecting the automatic speed limitation if the motor speedshould increase above synchronism and thus cause the relay H to pick up.

If the master controller is turned from point 2 to point 3 loweringafter relays H, R and contactor A have responded and the motor isrunning near synchronous speed according to Fig. 6 and characteristic D3in Fig. 7, no change occurs and the motor continues running in the samemanner, i. e., at a speed limited to approximately the synchronousvalue. However, when the master controller is moved from point 2 topoint 3 lowering before the speed limiting relay H drops out, forinstance, if the controller is rapidly moved from the off position orpoint I lowering to point 3, the circuit for coil 34 and contact 38 isopened in the controller MC before relay R is caused to pick up.Consequently, the motor is then connected in the same manner as at point2 lowering, except that relay R and contactor A cannot pick up. As aresult the motor circuit according to Fig. 5 is maintained and the motoroperates in accordance with the speed torque characteristic D2 in Fig. 7regardless of the lowering speed. Thus a load, if sufficientlyoverhauling, is permitted to accelerate downwardly to speeds abovesynchronism.

Reviewing the foregoing performance it will be recognized that inaccordance with the invention the resistor 4| has several essentialfunctions. When connected across the motor terminals TI and T3 (point Ilowering, Fig. 4, curve D| in Fig. 7) the resistor 4| forms part of abraking field circuit for single-phase braking performance. When seriesconnected in one of the pri-' mary phase leads, the resistor 4| servesto provide unbalanced three-phase energization for low torque operation(point I hoist, Fig. 2, curve HI in Fig. 7, and position 2 lowering,Fig. 5, curve D2 in Fig. 7). During the transfer from point I to point 2lowering (i. e., changing from the circuit of Fig. 4 to that of Fig. 5),the resistor 4| cushions and limits possible overload currents andprevents them from assuming short circuit magnitude. The resistor 4|also serves to provide one or more operating steps (position I hoist,Fig. 2) in addition to those obtainable by the change of secondaryresistance alone.

While these Various functions, without departure from the more generalfeatures of the invention, can be performed by several resistors, itwill be understood from the above-described embodiment that, inaccordance with a more particular feature of the invention, a singleresistor is sufiicient to take care of all of these functions.

As is generally the case in motor control systems of the above-describedtype, the number of speed positions in both running directions of themotor is more or less arbitrary and determined by the requirements ofeach particular application. Consequently, if desired, the number ofspeed steps in the hoisting or lowering direction may be changed. Forinstance, several single phase braking steps may be provided differingfrom each other by diiferent amounts of resistance in the rotor circuitor by having the resistor 4| inserted or shorted out of the primarycircuit.

A comparison of Fig. 5 with Fig. 6 shows that the relays forautomatically limiting the motor speed at point 2 lowering have theeffect or varying the amount of resistance effective on the primary sideof the motor and at the same time change the effective amount ofresistance in the secondary motor circuit. It is obvious that for somepurposes only an automatic change of resistance on the primary side oronly a change of the secondary resistance suifices. For instance, if thecontact 32 of contactor A in Fig. 1 is omitted only the secondaryresistance is reduced, and if the contacts 29 and 30 are omitted onlytheprimary circuit is affected.

As exemplified by the illustrated embodiment a system according to theinvention can be designed with a minimum of relays and contactors. As amatter of fact, the illustrated embodiment involves the operation ofelectromagnetic devices only at point 5 hoist and point 2 lowering.

It will be obvious to those skilled in the art after a study of thisdisclosure that the invention permits various embodiments andmodifications other than those specifically illustrated and describedWithout departing from the essential features of the invention andwithin the scope of the claims annexed hereto.

I claim as my invention:

1. An alternating-current motor control system comprising a three-phasemotor having three terminals, three line terminals, resistance means,control means attached to said motor terminals and line terminals andresistance means and having selectively adjustable control conditions,said control means connecting only two motor terminals to only tworespective line terminals and connecting said resistance means betweenone of said two motor terminals and the third motor terminal when in oneof said control conditions, and said control means connecting said threeline terminals to said respective three motor terminals and connectingsaid resistance means lac-

